Not all recipients were @hotmail.com. There were a fair range of different ISP.
ZA Advanced Email Protection has the maximum number of recipients set to 50, and number of emails sent to 5 in a time interval of 2 seconds.

Is this likely to be a ZA problem?

Or should I be looking at Outlook? or my wireless router? or Tiscali?

I sent the email again, separately to all 17 recipients. No error messages and have so far receibved two replies.